1. Field of the Invention
The present invention is related to a semiconductor memory device on which an electrically writable non-volatile memory is mounted.
2. Description of the Related Art
Conventionally, non-volatile memories have been necessarily employed as memories for storing thereinto system control programs, and memories used to store thereinto either parameters or data. Among these non-volatile memories, such memories which can be manufactured in the lowest cost are mask ROMs. However, data must be previously and physically formed in such mask ROMs in manufacturing steps of semiconductor memory devices, and thus, delivery appointed dates from definitions of ROM data up to product shipment become longer.
On the other hand, EPROMs are electrically-writable and ultraviolet-erasable memories. Since arbitrary values can be written at arbitrary addresses only one time, short appointed dates of delivery can be realized after ROM data has been defined. Also, all memory areas of such an EPROM can be erased by utilizing ultraviolet rays. However, when EPROMs are assembled in packages made of materials through which ultraviolet rays can not penetrate, ROM data may be merely written in EPROM, but cannot be erased.
Under such a circumstance, electrically writable/erasable programmable read-only memories (EEPROMs) have been recently used as major semiconductor memory devices. In memories which are writable/erasable in the unit of a word among EEPROMs, memory cell arrays must be subdivided every erase unit, areas of these memories are increased and these memories are not properly manufactured with large storage capacities in view of manufacturing cost thereof. Therefore, flash EEPROMs are presently utilized in many fields, since these flash EEPROMs are writable in the word unit and predetermined regions thereof are erased in a batch manner.
Since other EPROMs or flash EEPROMs are employed, short appointed delivery dates can be realized, and moreover, these memories may be employed so as to adjust production in the case that ROM data are required to be changed, depending upon shipping destinations. As a consequence, either EPROMs or flash EEPROMs have been mounted in most of the electronic appliances.
However, there are other memory utilities such as EEPROMs, namely rewriting operations are required plural times in the word unit. There are many possibilities that EEPROMs are mounted on systems in combination with either EPROMs or flash EEPROMs. For instance, this EEPROM is used in order to store there into an individual serial number, or an individual security code.
Data must be rewritten in a small unit, for instance, a serial number must be rewritten every time the present user is replaced by a new user, and a security code is rewritten in a periodic manner so as to increase a security level. In the case that EEPROMs are employed in such a memory utility, there are many opposites that necessary storage capacities of these EEPROMs are very small, for example, a flash EEPROM requires 8 Mbits, and an EEPROM requires 512 bits. When both a flash EEPROM and an EEPROM are mounted on the same chip, a substrate of a memory cell must be subdivided in order that this EEPROM can be individually erased in a small unit irrespective of the flash EEPROM. Both the EEPROM and flash EEPROM memories cannot be arranged within the same memory cell array, so that an area of a memory device is increased (refer to, for example, “Advanced Electronics I-9 VLSI memory” written by ITO Kiyoo, pages 23 to 24, No. 6 print in first version published by BAIFUKAN, Feb. 20, 2000.).
Under such a circumstance, there are some cases that the above-explained erasing operation may be realized by a computer program capable of controlling a flash EEPROM without mounting such an EEPROM. FIG. 18 indicates a process flow operation as to this computer program. In this process flow operation, a relevant erase block is firstly calculated from a rewriting address (step 131), and then, all of data of the erase block where the data which should be rewritten are present are stored in a RAM, or the like (step 132). An erasing operation of the above-described erase block is executed (step 133), and the data which have been stored in the RAM and correspond to the data to be rewritten are rewritten (step 134). Then, all of the data stored in the RAM are again written into the flash EEPROM (step 135). As a result, the rewriting operation can be carried out in the arbitrary word unit.
As previously explained, when the flash EEPROM and the EEPROM having the small storage capacity are mounted at the same time, if these flash EEPROM and EEPROM are mounted on the same chip in the mixing manner, then there is such a problem that the area of the memory device is increased. Also, in the case that the EEPROM having the small storage capacity is realized by the program control, lengthy time is required in the rewriting operation, and the erasing operation is required every time the rewriting operation is carried out. In a flash EEPROM whose rewriting time is limited, the following problem occurs. That is, the writing times of the region which is employed as the EEPROM are restricted to such value smaller than, or equal to the rewriting times of the flash EEPROM.